Reversible Luminescent Switching Induced by Heat/Water Treatment in a Zero-Dimensional Hybrid Antimony(Ⅲ) Chloride

Recently zero-dimensional (0-D) inorganic–organic metal halides (IOMHs) have become a promising class of optoelectronic materials. Herein, we report a new photoluminescent (PL) 0-D antimony(III)-based IOMH single crystal, namely [H2BPZ][SbCl5]·H2O (BPZ = benzylpiperazine). Photophysical characterizations indicate that [H2BPZ][SbCl5]·H2O exhibits singlet/triplet dual-band emission. Density functional theory (DFT) calculations suggest that [H2BPZ][SbCl5]·H2O has the large energy difference between singlet and triplet states, which might induce the dual emission in this compound. Temperature-dependent PL spectra analyses suggest the soft lattice and strong electron–phonon coupling in this compound. Thermogravimetric analysis shows that the water molecules in the lattice of the title crystal could be removed by thermal treatment, giving rise to a dehydrated phase of [H2BPZ][SbCl5]. Interestingly, such structural transformation is accompanied by a reversible PL emission transition between red light (630 nm, dehydrated phase) and yellow light (595 nm, water-containing phase). When being exposed to an environment with 77% relative humidity, the emission color of the dehydrated phase was able to change from red to yellow within 20 s, and the red emission could be restored after reheating. The red to yellow emission switching could be achieved in acetone with water concentration as low as 0.2 vol%. The reversible PL transition phenomenon makes [H2BPZ][SbCl5]·H2O a potential material for luminescent water-sensing.


Crystal Structure
The singe-crystal X-ray structure of [H 2 (Table S1). The final R 1 was 0.0411 (I > 2σ(I)), and wR 2 was 0.0777 (all data). There is one formular unit in the asymmetric unit ( Figure S1); that is, it consists of one [H 2 (Table 1), which are comparable to those in the literature [44]. Notably, there are abundant H-bonds (N−H···Cl and C−H···Cl H-bonds) among cations and anions, as listed in Table 2, which form a supramolecular layer structure, as shown in Figure 1d and Figure S2, where the lattice water molecules are located and form additional hydrogen bonds with Cl − ions of the anions within the layer ( Table 2). As shown in Figure 1e, the anions-packing adopts a topology of pcu (primitive cubic net) with a little distortion because of the low symmetry of structure and the distortion of the [SbCl 5 ] 2− polyhedrons. Noteworthy is that the water molecules are located around the center of cubes built by eight [SbCl 5 ] 2− units in the topology framework ( Figure 1e). C(4)-H(4A)···Cl (2) (6) Symmetry transformations used to generate equivalent atoms: #1 −x + 1, −y + 1, −z #2 −x + 1, y − 1/2, −z + 1/2 #3 −x + 1, −y + 1, −z + 1 #4 −x + 1, y + 1/2, −z + 1/2.   contains a NH 2 group only in the piperidine ring. Thus, the structural comparison made here highlights the importance of organic cations in constructing ionic metal halides with desired structure and property.

Photophysical Properties
To characterize the optical properties of [H 2 BPZ][SbCl 5 ]·H 2 O, the steady-state and time-resolved PL spectra were measured. The steady-state PL excitation spectrum (emission = 595 nm) shows two peaks at about 265 nm and 320 nm, suggesting 1 S 0 → 1 P 1 and 1 S 0 → 3 P 1 electronic transitions in Sb 3+ ions, respectively ( Figure 2a) [47]. The 1 S 0 → 1 P 1 electronic transition is allowed, while the 1 S 0 → 3 P 1 is partially allowed by the spin-orbit coupling [48,49]. However, the steady-state PL excitation spectrum (emission = 450 nm) only shows one peak at around 275 nm, suggesting that the 1 S 0 → 3 P 1 electronic transition nearly disappeared. Under the excitation of 290 nm, [H 2 BPZ][SbCl 5 ]·H 2 O exhibits dual-band broad emission peaking at 450 and 595 nm. The dual-band emission could be attributed to the exciton relaxation of 1 P 1 → 1 S 0 and 3 P n → 1 S 0 , respectively ( Figure 2b). However, there is only one broad mono-band yellow emission under the excitation of 320 nm ( Figure 2b); this triplet emission shows a peak centered at 595 nm with a Stokes shift of 275 nm. The title compound delivers a PLQY value of 14.33%, which is moderate among this class of compounds [26,30,31,49]. The time-resolved PL spectrum is shown in Figure 2c and utilized to calculate the PL lifetime of the title compound ( Figure 2c). The lifetime of [H 2 BPZ][SbCl 5 ]·H 2 O can be fitted well via the biexponential function (Equation (1)) [50]: The average lifetime can be calculated and obtained by the following Equation (2) [51]: After analyzing the steady-state and time-resolved PL spectra of the title compound, the PL mechanism is proposed as shown in Figure 2d [40]. Under excitation, the electrons in ground state 1 S 0 orbitals are excited to the singlet excited state 1 P 1 and triplet excited state 3 P n orbitals. The intersystem crossing (ISC) from the singlet to triplet orbitals results in the strong triplet emission during the electronic relaxation. After analyzing the steady-state and time-resolved PL spectra of the title compound, the PL mechanism is proposed as shown in Figure 2d [40]. Under excitation, the electrons in ground state 1 S0 orbitals are excited to the singlet excited state 1 P1 and triplet excited state 3 Pn orbitals. The intersystem crossing (ISC) from the singlet to triplet orbitals results in the strong triplet emission during the electronic relaxation.
Furthermore, to explore the origin of the broad emission and large Stokes shift of the title compound, temperature-dependent PL spectra ranging from 80 K to 320 K were collected under an excitation of 320 nm. As shown in Figure 3a, b, the title compound shows weaker PL intensity and broadening of the emission band along with the increasing temperature. These temperature-dependent performances are comprehensible. Typically, with increasing temperature, there is enhancement of thermal vibrations resulting in a thermal quenching of PL. Whereafter, the temperature-dependent PL spectra under the 320 nm excitation are further analyzed to obtain several important physical parameters, including Huang-Rhys factor (S) and electron-phonon coupling energy (Γop). The S can be obtained by fitting the curve of FWHM versus T using the following formula (Equation (3)): where FWHM represents full width at half maximum, ℏ is Planck constant, ω is the phonon frequency, k is the Boltzmann constant, and T is temperature [52]. Furthermore, to explore the origin of the broad emission and large Stokes shift of the title compound, temperature-dependent PL spectra ranging from 80 K to 320 K were collected under an excitation of 320 nm. As shown in Figure 3a, b, the title compound shows weaker PL intensity and broadening of the emission band along with the increasing temperature. These temperature-dependent performances are comprehensible. Typically, with increasing temperature, there is enhancement of thermal vibrations resulting in a thermal quenching of PL. Whereafter, the temperature-dependent PL spectra under the 320 nm excitation are further analyzed to obtain several important physical parameters, including Huang-Rhys factor (S) and electron-phonon coupling energy (Γ op ). The S can be obtained by fitting the curve of FWHM vs. T using the following formula (Equation (3)): where FWHM represents full width at half maximum,h is Planck constant, ω is the phonon frequency, k is the Boltzmann constant, and T is temperature [52].  [49], S represents the hardness or softness of the crystal lattice. A small S value represents a hard crystal lattice, which is unfavorable for electron-phonon coupling under excitation [53].
in this work), and Γop is the electron-phonon coupling energy [54].  [49], indicating strong electron-phonon coupling in the title compound under the excitation (Figure 3d). Overall, the temperature-dependent PL spectra analysis suggests the soft lattice and strong electron-phonon coupling in the title compound.

Theortical Calculations
Density functional theory (DFT) calculations were performed to investigate the band structure and photoluminescent mechanism of [H2BPZ][SbCl5]•H2O. As shown in Figure  S5, the title compound shows a calculated direct band gap of 3.45 eV, which is very close to the experimental one of 3.25 eV ( Figure S3 and S). The DOS shows that the valenceband maximum (VBM) is mainly contributed by Sb 5s and Cl 3p and the conduction-band minimum (CBM) is mostly contributed by Sb 5p, C 3s and 3p (Figure 4a). The nearly dispersionless VBM indicates negligible electronic coupling between inorganic [SbCl5] 2− units; that is, the title compound behaves a localized electronic structure [55]. Accordingly, the highest occupied molecular orbital (HOMO) is occupied by the inorganic moiety of [SbCl5] 2− mostly. The electronic cloud was round-like referring to the s electrons for Sb atom and spindle-like referring to the p electrons for Cl atom (Figure 4b). The lowest occupied molecular orbital (LUMO) is occupied by Sb atoms and conjugate electrons in benzene rings in organic [H2BPZ] 2+ cations (Figure 4c). The spindle-like electronic cloud of p To further discuss electron-phonon coupling interactions, the Toyokawa equation (Equation (4)) is used to fit the temperature-dependent PL FWHM: where Γ 0 represents the intrinsic line width at absolute 0 K (replaced by the data at 80 K in this work), and Γ op is the electron-phonon coupling energy [54]. The Γ op is fitted as 262. 36 [49], indicating strong electron-phonon coupling in the title compound under the excitation (Figure 3d). Overall, the temperature-dependent PL spectra analysis suggests the soft lattice and strong electron-phonon coupling in the title compound.

Theortical Calculations
Density functional theory (DFT) calculations were performed to investigate the band structure and photoluminescent mechanism of [H 2 BPZ][SbCl 5 ]·H 2 O. As shown in Figure S5, the title compound shows a calculated direct band gap of 3.45 eV, which is very close to the experimental one of 3.25 eV (Figures S3 and S4). The DOS shows that the valenceband maximum (VBM) is mainly contributed by Sb 5s and Cl 3p and the conduction-band minimum (CBM) is mostly contributed by Sb 5p, C 3s and 3p (Figure 4a). The nearly dispersionless VBM indicates negligible electronic coupling between inorganic [SbCl 5 ] 2− units; that is, the title compound behaves a localized electronic structure [55]. Accordingly, the highest occupied molecular orbital (HOMO) is occupied by the inorganic moiety of

Powder X-ray Diffraction and Thermogravimetric Analysis
The purity and the stability of the title compound were measured by powder X-ray diffraction (PXRD) and thermogravimetric (TG) analysis, as shown in Figure 5. The experimental PXRD pattern of the [H2BPZ][SbCl5]•H2O powders obtained by grinding the crystals is in agreement with the simulated one (Figure 5a), suggesting the purity and uniformity of the as-synthesized sample. Of note is that the as-synthesized crystals of [H2BPZ][SbCl5]•H2O could be steadily stored under ambient conditions for a long time (e.g., one month), as verified by PXRD (Figure 5a). The result implies that the water molecular is stable in the crystal lattice and no phase-transition would happen at ambient conditions. The moderate steric hindrance of organic cations can construct a 2D supramolecular framework, which endows  Figure  5b). That means the chemical formula is [H2BPZ][SbCl5] for the dehydrated phase. However, the corresponding PXRD pattern after losing water molecules differs considerably from the one of the pristine and the simulated one ( Figure S6), implying a slightly changed ionic structure after the loss of water molecules [24]. The second weight loss from 90 to 340 °C corresponds to a total decomposition of the title compound.

Powder X-ray Diffraction and Thermogravimetric Analysis
The purity and the stability of the title compound were measured by powder X-ray diffraction (PXRD) and thermogravimetric (TG) analysis, as shown in Figure 5. The experimental PXRD pattern of the [H 2 BPZ][SbCl 5 ]·H 2 O powders obtained by grinding the crystals is in agreement with the simulated one (Figure 5a), suggesting the purity and uniformity of the as-synthesized sample. Of note is that the as-synthesized crystals of [H 2 BPZ][SbCl 5 ]·H 2 O could be steadily stored under ambient conditions for a long time (e.g., one month), as verified by PXRD (Figure 5a). The result implies that the water molecular is stable in the crystal lattice and no phase-transition would happen at ambient conditions. The moderate steric hindrance of organic cations can construct a 2D supramolecular framework, which endows [H 2 (Figure 5b). That means the chemical formula is [H 2 BPZ][SbCl 5 ] for the dehydrated phase. However, the corresponding PXRD pattern after losing water molecules differs considerably from the one of the pristine and the simulated one ( Figure S6), implying a slightly changed ionic structure after the loss of water molecules [24]. The second weight loss from 90 to 340 • C corresponds to a total decomposition of the title compound.

Luminescent Water-Sensing
The water molecules could be removed by heat treatment of the title compound according to the TG analysis ( Figure 5b). Thus, we have performed the dehydration of the title compound by heating at 100 °C for 30 min. After dehydration, the single crystals show pulverization and are not transparent anymore. Moreover, the PL emission has been changed from yellow to red. Interestingly, the PL of the dehydrated sample could be recovered in ambient conditions (average 21 °C and 77% humidity in Fuzhou, China) quickly (inset of Figure 6a). To further characterize the PL emission switching, in situ PL spectra were performed for the dehydrated [H2BPZ][SbCl5] at ambient conditions. As shown in Figure 6a [SbCl5] to trace the water content in an organic solvent. Here, the dehydrated powder was soaked into acetone with different water contents (From 0-0.4% v/v). As the water concentration increased from 0.1 vol% to 0.2 vol%, the red-emissive compound turned to emit yellow light. The results show a detection limit of ca. 0.2 vol% for luminescent humidity-sensing for [H2BPZ][SbCl5], which is lower than that of another 0-D Sb 3+ -based IOMH of (PPZ)2SbCl7·5H2O (1.5 vol%) [30].

Luminescent Water-Sensing
The water molecules could be removed by heat treatment of the title compound according to the TG analysis ( Figure 5b). Thus, we have performed the dehydration of the title compound by heating at 100 • C for 30 min. After dehydration, the single crystals show pulverization and are not transparent anymore. Moreover, the PL emission has been changed from yellow to red. Interestingly, the PL of the dehydrated sample could be recovered in ambient conditions (average 21 • C and 77% humidity in Fuzhou, China) quickly (inset of Figure 6a). To further characterize the PL emission switching, in situ PL spectra were performed for the dehydrated [H 2 BPZ][SbCl 5 ] at ambient conditions. As shown in Figure 6a, 5 ] to trace the water content in an organic solvent. Here, the dehydrated powder was soaked into acetone with different water contents (From 0-0.4% v/v). As the water concentration increased from 0.1 vol% to 0.2 vol%, the red-emissive compound turned to emit yellow light. The results show a detection limit of ca. 0.2 vol% for luminescent humidity-sensing for [H 2 BPZ][SbCl 5 ], which is lower than that of another 0-D Sb 3+ -based IOMH of (PPZ) 2 SbCl 7 ·5H 2 O (1.5 vol%) [30].

Luminescent Water-Sensing
The water molecules could be removed by heat treatment of the title compound according to the TG analysis ( Figure 5b). Thus, we have performed the dehydration of the title compound by heating at 100 °C for 30 min. After dehydration, the single crystals show pulverization and are not transparent anymore. Moreover, the PL emission has been changed from yellow to red. Interestingly, the PL of the dehydrated sample could be recovered in ambient conditions (average 21 °C and 77% humidity in Fuzhou, China) quickly (inset of Figure 6a). To further characterize the PL emission switching, in situ PL spectra were performed for the dehydrated [H2BPZ][SbCl5] at ambient conditions. As shown in Figure 6a [SbCl5] to trace the water content in an organic solvent. Here, the dehydrated powder was soaked into acetone with different water contents (From 0-0.4% v/v). As the water concentration increased from 0.1 vol% to 0.2 vol%, the red-emissive compound turned to emit yellow light. The results show a detection limit of ca. 0.2 vol% for luminescent humidity-sensing for [H2BPZ][SbCl5], which is lower than that of another 0-D Sb 3+ -based IOMH of (PPZ)2SbCl7·5H2O (1.5 vol%) [30].   Single-Crystal X-Ray Diffraction (SCXRD): A suitable single crystal was selected under an optical microscope for SCXRD measurement. Intensity data were collected on a Supernova CCD diffractometer using graphite-monochromated Mo K α radiation (λ = 0.71073 Å) at 293 K. The structure was solved by direct methods and refined by fullmatrix least-squares on F 2 using the SHELX-2018 program package [57]. The non-hydrogen atoms were refined anisotropically. The hydrogen atoms in the [H 2 BPZ] 2+ cations were located at geometrically calculated positions, while those of the lattice water molecule were located from difference-Fourier maps and their atomic positions were refined. The crystallographic data and details for structural refinements are listed in Table S1. Selected bond lengths and angles are listed in Table 1. The hydrogen-bonding data are listed in Table 2. CCDC No. 2220612 contains the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif (accessed on 18 November 2022).

Reagents
Fourier Infrared Spectroscopy (FTIR): FTIR spectrum was measured by an instrument of Vertex 70 FTIR. Detailed data are shown in Figure S7.
Powder X-Ray Diffraction (PXRD): The experimental PXRD patterns were measured by a Rigaku Miniflex-II diffractometer by utilizing Cu K α radiation (λ = 1.54178 Å) at 30 KV and 15 mA in the angular range of 2θ = 5-65 • . The experimental PXRD pattern after the loss of water molecules were measured by the X-ray diffractometer with D8 Advance made by Bruker at 40 KV and 40 mA in the angular range of 2θ = 5-65 • . The simulated PXRD pattern was calculated using the SCXRD data via Mercury software.
Thermogravimetric Analysis (TGA): TG curve was recorded on a NETZSCH STA 449F3 instrument with a heating rate of 10 K min −1 under a dry N 2 atmosphere.

Solid-State UV-Visible Absorption Spectroscopy (UV-vis):
The solid-state diffuse reflectance data were recorded on a Shimadzu 2600 UV-vis spectrometer at room temperature (RT) in the range of 800-200 nm. The BaSO 4 plate was utilized as a standard that possesses 100% reflectance. The absorption data were then obtained from the reflectance spectrum using the Kubelka-Munk function α/S = (1 − R) 2 /2R, where α refers to the absorption coefficient, S refers to the scattering coefficient and R refers to the reflectance [58]. The test was performed on the solid-state sample in polycrystalline form.
Steady-State Photoluminescence Spectra: The photoluminescence excitation (PLE), photoluminescence (PL) spectra and photoluminescent quantum yield (PLQY) were measured on the FLS1000 UV/V/NIR fluorescence spectrometer. The excitation light source is a solid picosecond diode exciter with a pulse width of 57 picoseconds. The tests were performed on solid-state samples in polycrystalline form.
Time-Resolved Photoluminescence Spectra: Time-resolved PL spectra were measured on the FLS1000 UV/V/NIR fluorescence spectrometer. The tests were performed on solid-state samples in polycrystalline form.
Temperature-Dependent Photoluminescence Spectra: Temperature-dependent PL spectra were measured on the FLS980 fluorescence spectrometer ranging from 80 K to 320 K. The tests were performed on the samples in polycrystalline form.
Density functional theory (DFT) calculations: DFT calculations were implemented in the Vienna Ab initio Simulation Package (VASP) [59][60][61]. A generalized gradient approximation (GGA) for the exchange-correlation term with Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional was applied for the electron-electron exchange-correlation processes. The projected augmented wave (PAW) potentials were used with the valence states 2s, 2p for C and N; 3s, 3p for Cl and 5s, 5p for Sb, respectively. To ensure sufficient accuracy, the Brillouin zone was implemented by a Monkhorst-Pack k-point mesh of 3 × 3 × 5, and a high cut-off energy of 500 eV for the plane wave expansion was chosen. The self-consistent field (SCF) computations were set to a convergence criterion of 1 × 10 −5 eV and the force criterion was 0.02 eV/Å. The Fermi level (EF = 0 eV) was chosen as the reference of the energy.

Conclusions
In summary, a new PL 0-D IOMH of [H 2 BPZ][SbCl 5 ]·H 2 O has been studied in this work. The structure of this water-containing metal halide single crystal was determined in detail. The photophysical dynamics were investigated by temperature-dependent PL spectra and DFT calculations. The yellow emission of [H 2 BPZ][SbCl 5 ]·H 2 O originates from the recombination of singlet/triplet dual-band emission. DFT calculations show that the VBM and CBM are mostly located at the [SbCl 5 ] 2− unit, resulting in a large energy difference between the singlet and triplet states. Several important parameters, including S Γ op , have been fitted from temperature-dependent PL spectra. The results reveal that the large FWHM of PL should be owing to the soft lattice and strong electron-phonon coupling. It is worth noting that the water molecules in the [H 2 BPZ][SbCl 5 ]·H 2 O structure can be removed by heating, which causes the luminescent color change from yellow to red. The removed water molecules can be quickly restored to the material in the high RH environment or solution with water concentration as low as 0.2 vol%. This study not only provides a new type of lead-free metal halide-based emitter, but also paves the way for designing new PL metal halide materials for humidity-sensing.